Small-scale heterogeneities in depleted mantle sources: near-ridge seamount lava geochemistry and implications for mid-ocean-ridge magmatic processes

Abstract

The Lament seamounts form a chain of five volcanoes extending northwestward from the flanks of the East Pacific Rise (EPR) near 10° N (Fig. 1). They have been investigated using Sea MARC I and Sea Beam sonar1 and most recently with the submersible Alvin2. During the most recent field programme we collected precisely located samples from stratigraphic sections on caldera and crater walls of each volcano and from the adjacent EPR. Major objectives of this programme were to determine whether the seamounts and the nearby ridge crest shared common mantle sources and to elucidate off-axis magma storage and ascent processes. The chemical compositions of Lamont seamount lavas, which include some extremely primitive basalts, indicate that each sea-mount was fed from one or more closely spaced mantle sources with depleted chemical characteristics. The geochemical and isotopic data imply that there is no direct magmatic link between Lamont seamount and EPR accretionary magma sources. Lateral distances between individual seamounts, as well as those between the EPR axis and cones at the base of Sasha seamount are small (Fig. 1), leading us to the conclusion that chemical heterogeneities in depleted mantle sources exist on a very small scale (<5 km).